Next i choosing what part of the picture you want as your phone image, drag a box round the section of the image you want, you can move the box and resize it if you are not happy with the position when you first drag it, once you are happy with the position right click in the box and click on crop (remember...no matter how big or small you make that box the proportions will stay the right size for the phone)

and when you get to the resize box (as shown) type in the same proportions as you did in the crop tool bar into the top boxes and make sure the boxes on the right say PIXELS, and also check that you have the constrain proportions box ticked.